1. Field of the Invention
The present invention relates to a picture feature extraction system and a picture quality evaluation system. More specifically, the present invention relates to a picture feature extraction system and a picture quality evaluation system capable of reducing the amount of the information of a picture feature for evaluating picture quality degradation caused by video transmission.
2. Description of the Related Art
There is known a method for obtaining picture quality degradation resulting from compression coding applied while transmitting a video, a transmission error, or the like, by comparing picture feature so as to monitor an operating state of a transmission system in a video transmission service. The inventor of the present invention proposed picture quality monitoring or picture quality evaluation systems capable of highly accurately evaluating picture quality degradation by representing the picture quality degradation based on an index of mean square error (“MSE”) or a peak-to-noise ratio (“PSNR”) in the following Patent Literatures 1 to 3.
When a transmitting-end signal is x(t), a receiving-end (degraded) signal is y(t), and a total number of pixels is N, the MSE is defined by the following Equation (1).MSE=(1/N)·Σ[x(t)−y(t)]2  (1)
In the Equation (1), symbol t denotes two-dimensional coordinate composed of a horizontal coordinate element and a vertical coordinate element.
The MSE is estimated by causing each of a transmitting end and a receiving end to divide a picture into a plurality of blocks, to extract a coefficient of an arbitrary component in an orthogonal transform coefficient obtained by orthogonal transform, to transmit the extracted coefficient to a central monitoring chamber as a picture feature, and by comparing the transmitting end picture feature with the receiving end picture feature. In addition, the PSNR is derived from an error detection rate which is calculated when an invisible marker buried in the picture is detected.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2003-87823
Patent Literature 2: Japanese Patent Application Laid-Open No. 2003-9186
Patent Literature 3: Japanese Patent Application Laid-Open No. 2002-247606
However, the conventional techniques disclosed in the Patent Literatures 1 and 2 have the following disadvantages. Since each of the techniques is based on the comparison of the picture feature, it is necessary for both the transmitting end and the receiving end to extract the fair amount of the information of the picture feature so as to ensure high accuracy, and to transmit the picture feature to the central monitoring chamber over monitoring lines. As a result, for standard television (“SDTV”) video transmission at a standard line rate of three to six megabits per second (Mbps), for example, lines at a rate of several hundreds of kilobits per second are necessary as the monitoring lines for transmitting the picture feature.
In theory, it is possible to reduce the amount of the information of each picture feature to several tens of kilobits per second by assuming one frame as one block, and subjecting each block to spread spectrum and orthogonal transform. To do so, however, a memory capacity necessary to extract the picture feature and a calculation volume of the orthogonal transform are increased, thereby complicating an apparatus. For this reason, it is difficult to actually realize the reduction of the picture feature by the spread spectrum and the orthogonal transform.
Further, a method of reducing the amount of the information of the picture feature by dividing a video signal into a plurality of small blocks, and subjecting each block to spread spectrum and orthogonal transform, thereby reducing a calculation volume, and by rounding off an orthogonal transform coefficient to an arbitrary bit length (quantizing the orthogonal transform coefficient) may be considered. However, if the orthogonal transform coefficient is rounded off to about one to two bits, picture evaluation accuracy is disadvantageously deteriorated to an impracticable degree.
The conventional technique disclosed in the Patent Literature 3 is intended to make it unnecessary to provide the picture feature transmission monitoring lines, and to perform picture quality evaluation by burying and detecting an invisible marker. The invisible marker is information for the picture quality evaluation. Since the invisible marker is buried in a video signal, it is advantageously possible to dispense with the monitoring lines for transmitting the picture feature. Besides, since the invisible marker is buried for each block, a scale of a system can advantageously fall within an appropriate range.
However, the conventional technique disclosed in the Patent Literature 3 has the following disadvantages. The invisible marker is buried in the video signal at a lowest signal level which does not affect a human vision, and picture quality degradation caused by burying the invisible marker is only slight. However, even the slight picture quality degradation poses a problem on transmission of program materials between broadcast stations for which quite a high picture quality is required. In this respect, the conventional techniques disclosed in the Patent Literatures 1 and 2 are superior to that of the Patent Literature 3 since transmitted videos are not deteriorated according to the Patent Literatures 1 and 2. However, the conventional techniques disclosed in the Patent Literatures 1 and 2 disadvantageously need to reduce a rate of a transmission band for transmitting the picture feature down to the impractical level of several tens of kilobits per second, and to suppress the system from being complicated, as stated above.
The disadvantages of the conventional techniques disclosed in the Patent Literatures 1 and 2 are mainly due to an increase in the amount of the information resulting from extracting a plurality of bits (normally eight to ten bits) from each block as the picture feature so as to ensure sufficiently high accuracy.